چارچوب های آلی فلزی برای حذف فلزات سنگین از آب Metal–organic frameworks for heavy metal removal from water

1. Introduction 1.1. Sources of heavy metals in water Geological sources: Heavy metals (that is metals with density over 5 g cm
3 such as arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc) [1] polluting our water is a rapidly growing global concern. These elements can be found within the environment – be it in water reservoirs, the atmosphere or soil – in excess, due to various anthropogenic actions. It is also important to note the natural sources of heavy metal pollution. These include all types of rocks (igneous, sedimentary and metamorphic), which, through their interactions with the surrounding environment (i.e., weathering, erosion, soil formation and the rock cycle in general), transport and redistribute heavy metals [2]. Heavy metals most commonly found in rock-forming minerals include those which most easily leach due to mineral weathering such as nickel, cobalt, manganese, zinc, copper, and vanadium, in addition to metals that have intermediate stability such as scandium, yttrium and other rare earth elements, all the way to uranium or hafnium, which can be found in zircon and are resistant to weathering. These elements are then concentrated when hot hydrothermal fluids permeate the rocks, inducing chemical reactions that cause precipitation of minerals and creation of ores [3]. Such deposits are often found within sedimentary rocks, which, due to their porous structure and high permeability, are well suited for storage of solids. Soils are another medium responsible for heavy metal storage. Heavy metals in soil are found in relocated rock debris, insoluble minerals and organic matter (i.e., the solid phases), as well as in the water and air trapped within the soil (i.e., the fluid phases). These solid and fluid phases interact with each other and various ions passing through the system [4]. The concentration and identity of heavy metals in soil is directly related to the type of rock that the soil originated from. Most heavy metals can be found within the third layer – or horizon – of soil called the B-horizon. This layer contains elements which were once dissolved within the upper layer (the A-horizon) and then underwent eluviation (movement of the dissolved material downwards or sideways) into the lower layer, where they were deposited [5]. The B-horizon attracts heavy metals because it has a high concentration of iron oxyhydroxides and clay which are able to absorb the cationic elements [3]. Surface waters (from springs and streams to lakes and rivers) can carry heavy metals over a large distance and their chemical composition varies depending on the geological features over/ around which they flow. Other factors contributing to the identity and concentration of heavy metals in surface water include biological, chemical and physical influences such as living organisms, adsorption from sediments or organic and inorganic matter, dilution and evaporation, redox potential, pH and finally temperature [3].